Recently, liquid crystal display devices have been widely used in clocks, electronic calculators, various home appliances, measuring equipment, panels for automobiles, word processors, electronic personal organizers, printers, computers, TVs, and the like. Typical examples of the types of liquid crystal display include a TN (twisted nematic)-mode display, a SIN (super-twisted nematic)-mode display, a DS (dynamic light scattering)-mode display, a GH (guest-host)-mode display, an IPS (in-plane switching)-mode display, an OCB (optically compensated birefringence)-mode display, an ECB (electrically controlled birefringence)-mode display, a VA (vertical alignment)-mode display, a CSH (color super-homeotropic)-mode display, and a FLC (ferroelectric liquid crystal) display. There has been a shift in the driving method used from a conventional static driving to a multiplex driving, which has been commonly employed. Recently, passive-matrix LCDs and active-matrix (AM) LCDs, which are driven using a TFT (thin-film transistor), a TFD (thin-film diode), or the like, have been widely employed.
As shown in FIG. 1, a common liquid crystal display device includes two substrates (1) each including an alignment film (4); a transparent electrode layer (3a) serving as a common electrode and a color filter layer (2), which are interposed between one of the alignment film and the corresponding substrate; and a pixel electrode layer (3b) interposed between the other alignment film and the corresponding substrate. These substrates are arranged so that the alignment films face each other, and a liquid crystal layer (5) is held therebetween.
The color filter layer is constituted by a black matrix, a red-colored layer (B), a green-colored layer (G), a blue-colored layer (B), and, as needed, a yellow-colored layer (Y).
The amount of impurities in a liquid crystal material constituting the liquid crystal layer is strictly controlled because any impurities remaining in the liquid crystal material would greatly affect the electrical characteristics of the display device. It is known that a material constituting the alignment film also affects the electrical characteristics of the liquid crystal layer because any impurities remaining in the alignment film would migrate into the liquid crystal layer due to the direct contact of the alignment film with the liquid crystal layer. Thus, the characteristics of the liquid crystal display device due to impurities contained in an alignment film material is currently being studied.
On the other hand, a material of the color filter layer, such as an organic pigment, as well as an alignment film material, is also considered to affect the liquid crystal layer due to impurities contained therein. However, the direct effect of a material of the color filter layer on the liquid crystal layer has been considered to be very small compared with the effect of the alignment film material since the alignment film and the transparent electrode are interposed between the color filter layer and the liquid crystal layer. However, the thickness of the alignment film is generally 0.1 μm or less, and the thickness of the transparent electrode, which is a common electrode disposed on the color-filter-layer-side, is generally 0.5 μm or less even in the case where the thickness of the transparent electrode is set large in order to increase electric conductivity. Therefore, it cannot be said that the color filter layer and the liquid crystal layer are in an environment where they are completely isolated from each other, and the impurities contained in the color filter layer, which migrate via an alignment film and a transparent electrode, may cause a reduction in a voltage holding ratio (VHR) of a liquid crystal layer and an increase in the ion density (ID) in the liquid crystal layer, which results in faulty display such as white missing pixels, alignment inconsistencies, and burn-in.
In order to address the faulty display caused by impurities contained in pigments constituting the color filter, a method (PTL 1) of controlling elution of the impurities into a liquid crystal by using a pigment such that the proportion of a substance extracted from the pigment with ethyl formate is set to be equal to or less than a specific value and a method (PTL 2) of controlling elution of the impurities into a liquid crystal by specifying a pigment contained in a blue colored layer have been studied. However, there is not a great difference between these methods and a method of simply reducing the amount of impurities contained in a pigment, and these methods provide unsatisfactory improvements in addressing the faulty display in the present situation in which progress has been made in purification techniques for pigments.
On the other hand, focusing on the relationship between organic impurities contained in the color filter and the liquid crystal composition, a method in which the degree of difficulty in dissolving the organic impurities in the liquid crystal layer is represented as a hydrophobicity parameter of liquid crystal molecules contained in the liquid crystal layer and the hydrophobicity parameter is controlled to be equal to or more than a predetermined value has been disclosed. Furthermore, based on the correlation between the hydrophobicity parameter and a —OCF3 group at the end of the liquid crystal molecule, a method (PTL 3) of preparing a liquid crystal composition including a certain proportion of a liquid crystal compound having a —OCF3 group at the end of the liquid crystal molecule has been disclosed.
However, the essence of the invention disclosed in the cited document is reducing the effect of impurities contained in a pigment on the liquid crystal layer and there was no study on the direct relationship between the structure of a coloring material, such as a dye or a pigment, used for a color filter and the structure of a liquid crystal material. Thus, the issue of faulty display of liquid crystal display devices, which are becoming more advanced, has not yet been addressed.